Heat the milk, water, butter, salt and sugar in a saucepan. While cooking, add the flour and mustard powder. Mix well and beat vigorously with a whisk. Continue until the dough forms a ball and a white "carpet" appears on the bottom of the pan. Let it boil for a while, keep stirring, on a low heat until the flour is done (when the dough no longer smells like flour). Then turn off the heat and continue with the eggs, see below.
Continue mixing the dough with slow stirring to cool it down a bit. Add the eggs one at a time. Regularly scrape the edge of the bowl so that all ingredients mix well. When the dough is smooth and shiny, put it in a bag.
Preheat the oven to 185°C.
Pipe profiteroles of about 9 g on a baking tray lined with baking paper or a Silpat® mat. Sprinkle with grated cheese and bake for 16 minutes. Do not open the oven during baking, otherwise the profiteroles will lose their volume. Let rest for 10 minutes and then poke holes in the bottom of the puffs to fill with the sauce.